Pebble bed heat exchanger



Jan. 6, 1970 w; ZOZOW K. ET AL 3,488,042

PEBBLE BED HEAT EXCHANGER Filed Jan. 25, 1968 :s Sheet-Sheet 1 a 6, 1970 w. aR'zozowsKr ET AL. $488,042

PE BLE BEDHEAT EXCHANGER v Filed Jan. 25. 1968 3 s t s z Jan. 6, 1970 Filed Jan. 25, 1968 w. sazozowsm T AL PEB-BLE BED HEAT EXCHANGER 3 Sheets-Sheet 3 United States Patent 93,497 Int. Cl. FZEid 15/00; F23c 1/12; F2311 21/00 US. Cl. 263-19 6 Claims ABSTRACT OF THE DISCLOSURE In a pebble bed heat exchanger the junction assembly interposed between an upper enclosure through which is circulated an upward flow of heating gas and a lower enclosure through which is circulated an upward flow of gas to be heated comprises a communication spout constituted by a plurality of refractory elements divided into sectors; the elements are supported by separate vaults for transferring their weight to a support structure, a communication between the lower enclosure and a passage through which the heated gases are discharged, and a second vault which is placed above said lowermost vault and constitutes a thermal shield.

This invention relates to heat exchangers of the type comprising a moving bed of spherical pebbles of refractory material, and more especially to the junction assembly provided in a heat exchanger of this type between the upper enclosure through which is circulated an upward flow of heating gas and a lower enclosure through which is circulated an upward flow of gas to be heated.

Heat exchangers of the so-called pebble bed type have already been employed, notably in the petroleum industry. A general description of such an exchanger can be found on page 958 et seq. of Perrys Chemical Engineers Handbook (4th edition, McGraw-Hill). These heat exchangers are made up of two superposed enclosures connected by means of a junction assembly which forms a downspout having a smaller diameter than that of the enclosures and through which the spherical peb bles pass from the upper enclosure to the lower enclosure. The pebbles pass downwards through the upper enclosure and, during their downward motion, absorb the heat yielded by an upwardly flowing heating gas which is supplied from external heat sources through openings which are placed at the bottom of the upper enclosure. The pebbles which are heated in this manner move down through the spout under the action of gravity and thus reach the lower enclosure while yielding to an upfiowing gas stream the heat which has previously been absorbed.

The junction assembly must obviously be designed to prevent any mixing of the heating gas and heated gas, the pressures at which said gases are circulated being maintained at substantially equal values by means of pressure-regulating pipes which open on each side of the junction assembly. In order to prevent any accidental variation of pressure within an enclosure from giving rise to contamination of the heated gas by the heating gas or to a reduction in efficiency, it is found necessary to endow the spout with a small diameter and/or a substantial length in order that the pebbles contained therein should give rise to a pressure drop which is sufficient to prevent gas transfer processes.

The application of a heat exchanger of this type to the transfer of heat between a heating gas which penetrates therein at very high temperature and which is derived, for example, from a magnetohydrodynamic generator and a secondary gas which is also heated to a high 3,488,042 Patented Jan. 6, 1970 temperature presents a number of difliculties. In particular, operation under substantial loads of pebbles sets problems relating to the thermal and mechanical behaviour of the junction assembly and, in the devices which have been employed up to the present time, results in a high degree of abrasion of the spout by the pebbles.

The aim of this invention is to provide a junction as sembly which meets practical requirements more elfectively than comparable devices proposed heretofore, especially from the point of view of operation with gases at very high temperature such as those which are produced by a magnetohydrodynamic generator, this result being achieved in particular by providing a solution to the thermal and mechanical problems which are related to such high temperatures.

The invention accordingly proposes, in a pebble bed heat exchanger which consists of a moving bed of spherical pebbles of refractory material and which is designed for use with gases at very high temperature, a junction assembly interposed between an upper enclosure through which is circulated an upward flow of heating gas and a lower enclosure through which is circulated an upward flow of gas to be heated, said junction assembly being essentially distinguished by the fact that it comprises a communication spout constituted by a plurality of refractory elements divided into sectors each delimiting a vertical passage in coincident relation and applied against separate vaults for transferring their weight to a support structure, the lowermost vault being pierced by passages providing a communication between the lower enclosure and a passage through which the heated gases are discharged, and a second vault which is placed above said lowermost vault and constitutes a thermal shield.

The invention also consists in other arrangements which can usefully be employed in conjunction with the foregoing but can also be employed separately. A better understanding of the invention will be gained from the following description, reference being made to the accompanying drawings, in which:

FIG. 1 is a very diagrammatic sectional view taken along the vertical mid-plane and showing the general arrangement of a pebble bed heat exchanger;

FIG. 2 shows a junction assembly in accordance with one embodiment of the invention which is suitable for use in the heat exchanger of FIG. 1 and which is shown in cross-section along the vertical mid-plane;

FIG. 3 is an overhead view of the bottom vault of the assembly of FIG. 2;

FIG. 4 is a partial sectional view of another embodiment of the junction assembly.

The heat exchanger which is shown diagrammatically in FIG. 1 comprises two superposed vertical cylindrical towers A and B which communicate with each other via a junction assembly C. The upper tower A is provided with an internal lining 10 of refractory material which is intended to withstand high temperatures and is provided at the lower end thereof with an annular chamber 12 formed by a vessel or outer wall 14 and by an inner partition 16. Ducts 18 for the admission of hot gases supplied from combustion chambers (not shown) open tangentially into said annular chamber. A communication is established between the annular chamber and the lower portion of the tower A by means of a series of radial passages 20.

The hot gas which penetrates through the passages 20' rises within the tower and passes out through a top duct 22 along a path indicated by the arrows f,,.

The lower tower B which is separated from the upper tower by the junction assembly C is provided at the lower end thereof with a duct 24 for the admission of cold gas (such as air, for example). Said duct opens into an annular chamber 26 which is delimited by a grid 28. The cold gas to be heated rises in the direction of the arrows f and is discharged from the lower tower via a duct 32 after having passed through a vault 30 of refractory material. Pipes 34 and 36 are provided for the purpose of adjusting the pressures.

Spherical elements or so-called pebbles formed of refractory material (stainless steel or ceramic material, for example) are caused to circulate downwards throughout the heat exchanger. Said pebbles are fed into the heat exchanger through an inlet nozzle 38 in the upper tower A and collect in this latter, then pass into the lower tower B through a channel or spout which is delimited by the junction assembly C, and are finally discharged through a hopper which constitutes the bottom portion of the tower B. Said hopper is connected to a return system D which is provided with an elevator (not shown) for the purpose of returning the pebbles to the inlet nozzle 38 via an adjustable feed device. The pebbles thus follow the path which is indicated in FIG. 1 by the arrows F.

The junction assembly in accordance with the invention, as illustrated in FIG. 2, is intended to be employed in a heat exchanger of the type shown diagrammatically in FIG. 1. For the sake of enhanced clarity, the components of FIG. 2 which correspond to those already shown in FIG. 1 are provided with the same reference numerals, the pebbles having been omitted from the figure.

The lower portion of the upper tower A, which comprises an outer metallic supporting wall 40 and an internal lining of bricks formed of refractory material, is supported on a base plate 44 by a bottom section 42 consisting of a number of layers of refractory bricks, said support plate 44 being designed to carry the weight of the elements which are placed above it and to retransmit said weight to a support structure which is not shown in the figure. The support plate 44 is advantageously constituted by metallic plates which are cooled by a circulation of water through pipes 44' and 44".

An annular chamber is formed within the refractory lining of the lower portion of the upper tower A and passages formed in said lining provide a communication between said annular chamber and the base of the enclosure which is defined by the tower; at the base of said enclosure, the refractory lining defines a zone of frusto-conical shape which serves as a guideway to the spout.

In the embodiment Which is illustrated, the abovementioned passages 20 take the form of vertical gaps which are permitted to remain adjacent bricks.

In order to prevent the pebbles from passing towards the annular chamber 12 and to prevent this latter from being obstructed by said pebbles, the gaps between bricks are occupied by members which are capable of sliding relatively to each other (in order to prevent the appearance of mechanical stresses) and which delimit passageways of sufficiently small diameter; in the embodiment which is illustrated in FIG. 2, the members referred to consist of a pile of balls 46 having the same diameter and wedged into and retained in the gaps between the bricks. Provided that passageways of sufliciently small diameter are thus formed, said balls prevent the pebbles from escaping towards the chamber 12 during their downward motion along the tower A towards the junction assembly C. In practice, it is considered that there does not exist any danger of interpenetration as long as the diameter of the balls does not attain seven times the diameter of the pebbles. Since the passageways formed through the balls must remain as large as possible in order to reduce pressure drops, it is nevertheless preferable to maintain a dimensional ratio which closely approximates to the value mentioned. In another embodiment, the pasasges 20 can be replaced by a perforated wall of refractory material (FIG. 4) which performs the same function as the balls.

The hotgas generator or generators are adapted to communicate with the chamber through one or a number of admission "ducts '18 which open tangentially into the chamber 12'.

The spout which provides a communication between the enclosures" formed within the'towers A and B is composed of elements of're'fractory material in vertically stackedr'elation' so as todelimit a vertical duct for the flow of gases. In order to forestall any danger of fracture of 't hecon's'tituent elements of the spout as a result of thermal expansion"processesand'relief of internal stresses in the hot" state, each ofsaid' elements is made up of a pl rality of similar sectors each constituting an angular portion of*the eleme njtl The'top element 48' comprises a frusto-conical outer which is adapted as heat onth'e support plate 44 through'the' intermediary of insulating members 50. Said top elementl delimits an internal duct having a frustoconical inletportioni-which joined'to the frusto-conical wall formed at-th'e bottom of the uppertower A and to alower cylindrical portion. i

The, spout can'fcomprise any numben of elements greater than two. Thus, the spout which is illustrated in FIG. 2'is composed of three superposed elements. The central element 52 has an outer wall which is also frustoconical and is supported by said wall on a first domical supporting vault 54 which is constituted by an assembly of bricks of refractory material. Said domicalvault also serves as a thermal barrier between the support plate 44 and the gases which are heated to a high temperature and which are discharged from the lower tower into the outlet duct 32 via' a collector space 56, the top wall of which is formed by the domical vault.

Finally, the bottom element 58 of the spout has a shape which is similar to that of the central element and is supported by a second domical vault 60 which is constructed of refractory bricks. As shown in FIGS. 2 and 3, ducts 62 for the flow of hot gases are formed within said bricks. Said ducts are constituted by cylindrical bores formed in the bricks or preferably by the juxtaposition of recessed portions formed in the faces of the bricks.

The peripheral portion of the second domical vault carries the weight of the top domical vault by means of spacer members 64 of refractory material. The thickness of said spacer members is such as to provide a collecting space 56of. suificient height for the gases which pass out of the ducts 62(The above-mentioned peripheral portion of the second domical vault 60 rests on a second support plate 66 which is preferably constituted by metallic components cooled by a circulation of water andsaid second support plate. is in turn carried by a framework. The lower tower B has a structure which is similar to that of the upper tower A. v v

It is apparent that the presence of domical vaults which are each designed to support the constituent elements of ,the spout and the separation of said elements into sectors serve to prevent theflappearance of high stresses; The delimitation of passageways for the admission of .hot gas by means of a pile of balls which are capable of displacement with respect to each other prevents the pebbles from escaping towards the admission chamber without giving rise to the problems which would otherwise be presented .by expansion processes within massive parts.

What we claim is:

1. A-pebble bed exchanger utilizing a moving bed of spherical pebbles of refractory material comprising a refractory wall. defining an upper enclosure, an annular chamber which is disposed around the base of said upper enclosure and communicates,.therewith by way of admission passages for delivering-an upward flow of heating gas to said upper enclosure, a lower enclosure through which is circulated an upward flow of gas to be heated, and a spout for the vertical downward transfer of pebbles between the lower enclosure and the base of the upper enclosure beneath said admission passages wherein said spout comprises at least one lower refractory annular assembly projecting through and supported on a first vault which is formed with gas communication passages and carried at its periphery on a support structure and one central refractory annular assembly projecting through and supported on a second vault located above said first vault and supported on said support structure and defining therewith a collecting space for the heated gases flowing out of said lower enclosure and which con stitutes a thermal shield protecting against the action of the heated gases, a base plate supported on said support structure and supporting said refractory wall of the upper enclosure, said refractory assemblies being placed in superposed relation.

2. A pebble bed head exchanger in accordance with claim 1, wherein said admission passages are constituted by ducts which are partially obstructed by piled balls, having a diameter which is larger than the diameter of the pebbles and which form passageways which are of smaller dimension than the diameter of the pebbles.

3. A pebble bed heat exchanger in accordance with claim 1, wherein said admission passages are constituted by perforations formed in a refractory Wall and having transverse dimensions which are smaller than the diameter of the pebbles.

4. A pebble bed heat exchanger in accordance with claim 6 wherein said spout also comprises a top assemhly, all of said lower, central and top assemblies having a frusto-conical outer wall, the top assembly hearing on said base plate through a bed of refractory bricks.

5. A pebble bed heat exchanger in accordance with claim 1, wherein the periphery of the first vault supports the second vault.

6. A pebble bed heat exchanger in accordance with claim 1 including means for circulating cooling water in said base plate.

References Cited UNITED STATES PATENTS ROBERT A. OLEARY, Primary Examiner A. W. DAVIS, Assistant Examiner US. Cl. X.R. l65l07 

